1. Field of the Invention
The present invention relates generally to solar cells and, more particularly, to a passivated deep p/n junction obtained in a wafer by ion implantation.
2. The Prior Art
Currently, solar cells for the most part are made with shallow p/n junctions on the order of about 0.3 micrometer, which junction is highly doped. A p/n junction is required in a solar cell to collect the photogenerated carriers. A suitable wafer of one type, say p-type, is provided with an opposite type surface layer, say n-type, about 0.3 micrometer deep, to effect such a p/n junction between the wafer and the surface layer. The doping of this surface layer also needs to lower the surface sheet resistance of the wafer to levels low enough to allow for conduction of the photo-generated current carriers to a solar cell grid contact formed on the wafer's surface without appreciable ohmic losses thereat. In order to achieve this desired end, present day solar cell technology utilizes, for the most part, doping levels in excess of 10.sup.20 cm.sup.-3 so as to yield a wafer sheet resistance of less than 100 ohms per square.
A basic problem encountered with this present day solar cell technology is that in the case of doping levels in excess of 10.sup.20 cm.sup.-3, the lifetime of the minority photogenerated current carriers in the surface-doped region of the wafer (i.e., in the emitter region of the solar cell) is so short that an appreciable percentage of such minority photogenerated current carriers escape from being collected by the p/n junction due to rapid bulk recombination, such as doping-dependent Auger recombination. As a consequence, the conversion efficiency of the solar cell is adversely affected. Some workers in the field have attempted to alleviate this problem by making the p/n junction even shallower than 0.3 micrometer and increasing the level of doping in the emitter region still further. As known, a decrease in the p/n junction depth increases the sheet resistance. Such increased sheet resistance is, of course, compensated for by the heavier doping in the emitter region. Thus, sheet resistance is maintained below 100 ohms per square. Since the p/n junction is shallower, however, more photons penetrate the emitter, with the consequence that more current carriers are photogenerated in the base region of the cell, where they can be collected and conversely, less current carriers are photogenerated in the emitter region, where a large percentage thereof is lost. Since the p/n junction is very shallow however, more often than not reliable metallic electric contacts to the front surface of the solar cell cannot be effected. This problem of reliability is due to the fact that the metallic electric contacts must be made to react with the front surface of the solar cell so as to properly adhere thereto. When the p/n junction is too shallow, however, the metallization reaction on the front surface of the cell will penetrate this shallow emitter region, cross over the p/n junction and introduce deleterious shunt resistance into the solar cell.